354
CHEMICAL ENGINEERING
The movement of the rakes allows sufficient agitation to bring the washes in good contact with the crystals and the speed may be simply regulated to give sufl&cient time for the diffusion of the soluble matter from the interstices of the solids by the feed and rakes. Besides this it is a continuous operation requiring practically no attention.
Akins Classifier. — This machine is designed to accomplish separation in the same way as the Dorr classifier. In it the intermittently acting rakes are replaced by a continuously moving helix, the flights being interrupted at intervals to allow washing of the solids and return of the liquids.
Other Classifiers. — These include the Ovoca, using a continuous double helix; the Esperanza, using a continuously moving belt or chain equipped with rakes or paddles, and various other combinations of rakes and paddles, screws, etc. all based on the same principle. For definite particle-size classifications screens are more efficient than classifiers.
Continuous Counter-current Decantation. — This system is being quite generally used in the chemical world to replace the older intermittent agitation and decantation washing systems as well as more modern filter installations. It has as its basis the application of the Dorr thickener to remove suspended solids contin-
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RAW MATERIALS
WCAK SOUUTION
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FIG. 7. — Typical counter-current decantation.
uously from solutions as they settle out. The Dorr thickener as shown on p. 280 consists of arms equipped with plows attached to a central shaft in the settling tank so that as these arms rotate they rake the solids which have settled out to the central solids discharge. The clear liquor overflows into the launder.
The power consumption of such a machine is less than K horsepower for mechanisms, in tanks up to 40 ft. diameter. The labor required by six thickeners is one-half of one man's time per shift and on larger installations will be proportionately less per thickener. The size tank required is the theoretical area required to settle out the solids continuously which is considerably less area than required by an intermittent system due to the fact that the time-consuming period of decanting and removing the solids is done away with by continuous operation.
In Fig. 7 is shown a typical continuous counter-current decantation installation together with the continuous-agitation system as it would be applied to the manufacture of phosphoric acid from phosphate rock and other processes which is a representative leaching problem.
The wash water is added in thickener No. 3 and the overflow from No. 3 goes to No. 2 and hence to the agitator to be used in making up the original solution. The solids settling out in No. 1 thickener are pumped from it to No. 2 by means of diaphragm pumps and the solids from No. 2 to No. 3, etc. By this system therefore, the solids are progressively coming in contact with weaker and weaker solution and as discharged by the last thickener will contain practically no values.
The percentage of extraction obtained by an installation such as this can be readily computed having the following data. First to what final density will the solids settle,